The present invention relates to steering. More particularly, the invention relates to steering towed mobile vehicles by remote control.
As used herein, the term "axle" is defined as a shaft which connects wheels located on opposite sides of a frame which is supported by the shaft and by springs connected thereto. The term "short axle" is defined as a shaft which connects a pair of wheels located on the same side of the frame. The term "spindle" is defined as a short conical shaft on which a wheel is mounted.
The term "fixed axle" is defined as an axle the orientation of which remains unchanged with respect to a frame which the axle supports. The term "steerable axle" is defined as an axle which is capable of changing direction with respect to and independently of a frame which the axle supports. The term "fixed wheel" is defined as a wheel the orientation of which remains unchanged with respect to an axle on which the wheel is mounted. The term "steerable wheel" is defined as a wheel which is capable of changing direction with respect to and independently of a fixed axle to which the wheel is connected by a spindle.
Trucks, tractors, and trailers are well described in the Encyclopedia Britannica, volume 18, pages 721-723, hereby incorporated by reference.
Tractor-trailer combinations known as "eighteen-wheelers" present special problems. FIG. 1 shows a top plan view of a first type of eighteen-wheeler presently in use, generally denoted by the numeral 9. The frame 10 of a trailer 2 is supported near the by the frame 7 of a tractor 1. The frame 7 is supported by a fixed front axle 4 provided with a pair of steerable wheels 3 mounted on a pair of spindles 6, and by two fixed rear axles 4, each of which is provided with two pairs of fixed wheels 3a. The trailer frame 10 is further supported by two fixed axles 4 near the rear of the trailer 2. Each of these rear axles 4 is likewise provided with two pairs of fixed wheels 3a. A "fifth wheel" 8 is fastened to the frame 7 of the tractor 1.
There are serious disadvantages inherent in the eighteen-wheeler 9. The following examples are illustrative of the problems inherent therein.
(1) Because the only axles supporting the frame 10 of the trailer 2 are the two rear axles 4, approximately half the weight of the trailer 2 is borne by the frame 7 of the tractor 1. An immediate consequence of this burden on the tractor 1 is that tractors pulling eighteen-wheelers must be large, heavy, and rugged enough to bear about half the weight of the trailer and of the payload, as well as the weight of the tractor itself. Clearly, if part of this extra weight could be borne by the trailer, it would be possible to utilize lighter and less expensive tractors. Furthermore, the portion of the fuel utilized to move the tractor alone could be considerably reduced, leading to even greater savings and to a significant advantage in fuel economy and energy conservation.
(2) Because there are only fixed wheels 3a supporting the trailer 2, it is impossible for the trailer 2 to turn without some of the wheels 3a skidding or dragging to some extent. The farther apart the rear axles 4, the more serious is the problem of wheel drag. This problem causes loss of control, wasteful loss of rubber from the tires mounted on the wheels 3a, and wasteful loss of the extra energy required to overcome sliding friction as opposed to rolling friction. The trailer 2 is unstable in a turn. The faster the speed and the sharper the turn, the greater the instability.
(3) The fifth wheel 8 of the tractor 1 furnishes a leg to support the front end of the trailer 2. However, it does not furnish full support for the full width of the front end of the trailer 2. This deficiency causes the bed (not shown) of the trailer 2 to rock down on one side and up on the other side. The extent to which such rocking occurs depends on the turns that the trailer 2 makes, the imbalance of the load (which can be caused by shifting of the cargo in transit), road conditions, and other variables. This particular type of instability is almost continuous while the eighteen-wheeler 9 is in motion.
(4) Often the longitudional center of the trailer, when loaded, is in continual up-and-down motion. This motion contributes to the instability of the trailer 2, and may in extreme cases cause the trailer 2 to collapse at the longitudional center.
(5) The looseness of the trailer king pin (not shown) in the fifth wheel 8 of the tractor 1 causes some shock, which contributes to a deficiency in traction and to a rough ride for both cargo and driver.
(6) Tandem axles are not equivalent to two single axles. Transportation authorities do not allow as much weight to be carried by a pair of tandem axles as by a pair of single axles.
(7) Tandem axles on either the tractor 1 or the trailer 2, because they drag in a turn, can individually or in combination initiate a slide of the eighteen-wheeler 9 into a "jack-knife."
(8) The trailer 2 cannot independently furnish braking for the trailer and its payload (not shown). The push of the trailer 2 against the tractor 1 when brakes are applied to the tractor 1 causes the tractor 1 to yaw.
(9) There is nothing between the trailer 2 king pin (not shown) and the rear axles 4 of the eighteen-wheeler 9 to function as a load-bearing stabilizer for the trailer 2. It is part of the function of the massive tractor 1 to counteract the sideward pull of the trailer 2 upon the tractor 1 when the tractor changes direction at highway speeds. The sharper the turn and the faster the speed, the greater is the pull of the trailer 2 on the tractor 1.
(10) The existence of the problems just enumerated has caused the gradual emergence of heavier and more massive tractors 1. These massive tractors have larger engines. The role of the tractor 1 is to pull, to steer, and to give stable traction to the eighteen-wheeler 9 while carrying its cargo at highway speeds. The heavier tractors 1, along with Transportation Department load limits on tandem axles 4, cause the eighteen-wheeler 9 to carry less of a payload. The increased horsepower of the tractor 1 engines (not shown) causes less fuel efficiency. These factors, in combination, raise the cost per ton-mile of freight substantially.
In an attempt to alleviate these problems, prior-art devices have utilized steerable axles with fixed wheels for eighteen-wheelers; see, for example, U.S. Pat. Nos. 3,149,858, 3,533,644, 3,712,641, 3,734,538, 4,120,509, 4,244,596, and 4,463,966. Special devices have also been devised for steering goose-neck trailers, the devices including steerable short axles, wheels steered by friction against the road, and rear wheels forcibly steered from the fifth wheel of the tractor.
These prior-art devices are expensive to manufacture and maintain; and, because of their specialized design, of limited utility. Additionally, the devices and methods described in these patents are not effective for negotiating high-speed turns on highways and public streets. They are, moreover, difficult to connect and disconnect. More specifically, when steerable axles are used to make a turn, the base of support provided by the axle is diminished proportionately to the degree of the turn.
More recently, a significant advance in the technology was made by Ducote, as disclosed by U.S. Pat. No. 4,740,006. Ducote discloses a steering system for use in a tractor-trailer combination in which all trailer axles are load-bearing and fixed. The rearmost axle on the trailer has fixed wheel mountings, and serves as a pivot for turning the trailer when the trailer moves in either forward or reverse direction. The axle or axles located forward of the rearmost axle of the trailer and aft of the trailer king pin has or have directional wheels which automatically track and follow in concentric pattern the path of the rear wheels of the tractor. The force for steering the directional wheels emanates from a sensor nested in the V-slot of the fifth wheel of the tractor. The system is capable of being retrofitted to existing trailers, or optionally being used as equipment for trailers not yet manufactured. Ducote's patent is incorporated herein by reference.
While Ducote's invention represents a tremendous advance over existing technology for solving the problems enumerated above, there have since come into prominence trailers with spread-tandem axles. As stated above, the greater the spread of the tandem axles, the more serious the problem of wheel-drag becomes. Spread-tandem axles are conventionally separated by a distance of about ten feet. Such a trailer is shown in FIG. 2 and designated by the numeral 2a.
These rigid spread-tandem axles with fixed wheels add another dimension to turning, in that these spread axles must be dragged around to make the turn. The distance by which the tires and wheels are dragged depends on the individual axle loads and the road surface under each of the tires. In any case they must be dragged into alignment, for every turn that the rig makes. To compensate for this sideways dragging, the driver makes an extra-wide turn. So, in the first place the driver turns wide to accommodate the length of the trailer; and in the second place, the driver turns wider yet to have additional distance for the dragging and inability of the rigid tandems to turn. The reason that the rigid spread tandems are used is that they are allowed the axle weight of single axles. This allowable weight is 20,000 pounds per axle. This helps them get through the Department of Transportation weight scales. This severe problem is evident at the access and exit ramps of interstate highways.
These problems are always with the driver. When driving in cities, the driver must find an intersection at which the driver is able to make a turn. The turn that is made is seldom an easy turn; it is a turn that the driver is barely able to make. Thus the driver and the rig must roll until the driver is able to find a suitable intersection. The time spent finding this begins at the point that the vehicle is loaded, and ends at the point the load is discharged.
"Sliders" are rear-axle groups mounted as a truck which is slidable on the beams of the trailer. One function of the slider is to shorten the wheelbase when the rig is unable to back into a tight place for unloading. When the driver sees that he or she is unable to back into an unloading spot, the driver parks the rig, gets out of the cab, manually unlocks the slider, gets back into the cab, locks the wheels of the slider, then backs up a few feet to shorten the wheelbase. The driver parks the rig again, and gets out of the cab again, and locks the slider again in this new position under the trailer. He or she gets back into the cab a third time, and backs the rig to a spot where it can be unloaded. Though the driver has shortened the wheelbase, it is still usually not short enough. The driver will still run over curbs and medians, in order to get the rear of the trailer to the right spot.
"Extendables" are semi-trailers that telescope out to accommodate the length of reinforced concrete pilings, steel beams, and other structural members. These extended trailers have aggravated problems in turning because of their extra length.
Military semis are used to transport tanks and other military vehicles and equipment. These long semi-trailers have all of the problems of other long semi-trailers. Additionally, they are used in peaceful transport of military vehicles in old-world cities. The streets in many of these old-world cities are narrow, having been built for horse-drawn vehicles.
Transportation needs have caused semis to be built longer as time goes on. The longer the semi-trailer, the more difficult it is for the tractor-trailer 9a (FIG. 2) to get around an intersection or make a turn. There are many instances every day of tractor-trailers intruding in the adjacent lanes of traffic to make a turn. This is a dangerous practice, and disrupts traffic that is sometimes already heavy. Another problem is that the rear wheels of the trailer 2a ride over the curbs, sidewalks, and medians. This demolishes curbs and street signs, and brushes utility poles, which is damaging to both the trailer and the utility pole. This problem is particularly severe at intersections and ramps that lead to and from interstate highways.
Emergency vehicles and hook-and-ladder semis for firefighting have a greater problem with this situation because of the length of the semi-trailers.
To alleviate the problems arising from spread axles, dump valves are being used for one of the two axles on the spread. By exhausting the air springs of one axle, the axle no longer is under load and can travel through a tight curve more easily. Most spread-tandem platforms dump the air from the rearmost axle. By doing so, the platform has the maneuverability of a trailer that is significantly shorter.
Suspension manufacturers teach that the dump-valve system was intended for limited use, such as tight ninety-degree turns, backing into loading docks, maneuvering within a yard, and other "creep-speed" applications. But the improved maneuverability is leading some drivers to dump air when making moderate curves at speeds of about thirty miles per hour.
Accordingly, some trailer and component manufacturers are starting to object to what they consider abuse of the dump valve. The Spicer Trailer Products Division of Dana Corporation issued an engineering update stating that the company would not warrant the axles used on spread-axle trailers equipped with dump valves. In pertinent part, the update reads as follows:
"When a dump valve is actuated, the axle equipped with the dump system sees only its own weight and that of the attached suspension components, unless a regulator is used to maintain some air pressure in the air bag. The other axle sees the full load normally carried by both axles, causing severe overloading of that axle. Unless such dump valves can be provided to automatically apply air pressure above speeds of five mph, Spicer Trailer Products does not approve, and will not warrant axles used on spread axle trailers equipped with dump valve."
Great Dane does not offer dump valves on spreadaxle platforms for increasing maneuverability or reducing tire-scuffing. "We will put dump valves on both axles in order to control dock height, but we won't put them on just one axle," states Paul Crabtree, manager of engineering at Great Dane's platform plant in Memphis, Tenn. "We will put them on lift axles if a tandem is already in place, because we are assuming that the trailer will be operated with the axle lifted. The trailer will have to be built to handle that."
Clearly, these attempts by the prior art to solve problems associated with spread-tandem axles on eighteen-wheelers fall far short of providing satisfactory solutions to these problems.